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there is not info to be able to give a definite answer
to work it out
you have top know the diff ratio, wheel tyre circumference to be able to work from rpms of the engine to road speed
However as a pointer, most cars will be around 3200 rpms at 60mph as this will be direct drive and od causes a drop in rpms
Being an automatic find yourself a good section of low traffic road and start from stop slowly increase the rpms and watch the rpms. keeping the accelerator steady you will see when the auto changes by a drop in rpms. slowly increase road sped until you see the next drop in rpms and so on. Over drive normally comes in around 55 mph or around 3000rpms
I would think that it is working because 3500rpms at 75mph is reasonable rpms to be expected. If it wasn't working I would expect the rpms to be around 3800 to 4000.at 75mph
You have two problems. The surging is from a dirty throttle body and /or air filter. The selector light could be from a bad selector switch or internal transmission problems. Have the car scanned for codes.
on most newer vehicles the rev limiter is lower in park or neutral than in gear. i dont know why gm is doing this i just know that its normal. i dont know what year they started this but im guessing that is what your seeing. does it run fine in gear?
Hi Tom your vehicle is equipped with a Rev limiter it will not allow you to rev the engine above 4000 with the trans in park; as for the first problem you may have one of two things, one of the spark plugs or spark plug wires is bad or you have a flat spot on the throttle positioning sensor in which case have be replaced. Good luck?
on almost all gm cars after 96 the engines were governed to not free rev over 4000rpm. when in gear and power braking the rpms stay lower cause of the load on engine. when reving above 3500 rpms the computer cuts back the timing and fuel to reduce engine damage if left to free rev. hop that helps.